fixpath.icn: Program to replace path in a binary file

Usage:  fixpath filename oldpath newpath

Fixpath changes file paths or other strings in a binary file by modifying
the file in place.  Each null-terminated occurrence of "oldpath" is
replaced by "newpath".

If the new path is longer than the old one, a warning is given and the
old path is extended by null characters, which must be matched in the
file for replacement to take place.  This is dangerous in general but
allows repairing an errant fixpath command.